Osmosis
is the process by which cells try to
deal with inbalances caused by large
molecules in solutions.

Osmosis
is a process by which water moves
through a semi-permeable membrane to try
to equalize the concentration of a
solute (such as salt or sugar) on both
sides of the membrane.

What does
this mean?

Let's
explain this with pictures.

Here
we have two solutions.

Each one
has the same amount of water.

However,
the solution on the left has a lot of
sugar dissolved in it.

The
solution on the right has only a little
sugar in it.

Is this
fair?

The
permeable membrane is like a sieve.

Water can
go through it, but the big sugar
molecules cannot.

The water
molecules go to where there is lots of
sugar!Yum
yum!!!!

This will
be fair when there is the same amount
of sugar for each water molecule!

Now,
see how many of the water molecules have
gone through the permeable membrane to
the sugar- rich side.

There is
less water on the sugar-poor side now.

Sometimes
osmosis can make the two sides have an
equal concentration, but sometimes other
factors prevent that.

For
example, osmosis would not lift the
water up so high as the water level that
you see in the diagram

What is
important here is that you understand
the main idea.

Let's see
why understanding this is important for
understanding a little about how cells
work.

Try this
next example.

The
salt concentration inside the cell is
equal to the salt concentration in the
water.

Here
is a cell in the early oceans of
the world. There is salt
in the water outside the cell.
There is salt inside the
cell.

The
salinity or saltiness is the
same on both sides of the cell
membrane.

The
water outside the cell and the
water inside the cell are
equally salty. This is a
balanced situation.

Molecules
may pass through the cell
membrane, but about the same
number go in as go out.

It is saltier inside the cell than
outside.

Here we have a
higher concentration of salt
inside the cell than in the
water outside, See what
happens when you put a salt
water fish in fresh water.

The
salt cannot get out of the cell,
so water flows in to try to
equalize the saltiness.

The water moves
toward where the salt is.
There is more saltiness inside
the cell than there is outside
it. Water moves into the
cell to try to even this
out. Eventually there will be balance
-- or the cell will burst
-- and die.

The
cell is in very salty water now.

Here the cell
finds itself in very salty
water. The salt
concentration is greater outside
the cell than it is inside the
cell.

Water
leaves the cell to try to dilute
the salt solution.

The water moves
toward the salt. As it
moves out of the cell, the cell
grows smaller. It
shrivels. Is is becoming
dehydrated.

Humans use salt to to
preserve food by having the
water in the food go out to the
salt, for example, in salted
fish.

Also, natron is a salt used
to dry out mummies.

Natron is a kind of salt. The
Egyptians used
to pack the bodies of dead
pharaohs in natron (a kind of
salt) in order to dry out
their tissues and make mummies of their
remains.

The
first living cells had to stabilize
their interior chemicals despite the
changes in the physical world around
them. Only those that succeeded
lived and were able to reproduce
themselves..

Now how does this work with plants and
animals?

Plants

A plant cell has a cell
membrane that is inside a cell
wall. If the cell takes in a lot
of water, the membrane winds up pushing
against the stiff cell wall. The
cell wall stops the cell from absorbing
any more water.

When plant cells are
full of water, we say they are
turgid. It is this turgidity
that makes the grass stand up and the
leaves spread out in the sunshine.

When there is a shortage
of water, water may be released through
the cell membrane. The plant's
leaves start to wilt. The cell
membrane pulls away from the cell wall,
but it is still protected by it and so
often can take up water again when water
becomes available. Plant
cells have a structure called a vacuole
which is like a bubble. When water
is withdrawn from the cells, the vacuoloe
expands and helps the cell structure to
remain intact.

Animals

Animals are more
vulnerable than plants to changes of
water in their cells. If they take
in too much water, the cell membrane
just breaks, as there is no cell wall to
support it. Animals have kidneys
to keep the water and salts in their
bodies in balance with the normal state
of their cells.

I have described what
happens with salt solutions, but the
same thing happens with sugar
solutions. Life is a
chemical balancing act. The
animals and plants evolve ways to keep
their internal environments functioning
well. Changes in the external
environment in which we all live pose
continuing challenges to living
organisms.